Receive diversity is used as the technique for realizing improvement of quality of communication service and expansion of the network capacity. This receive diversity has a plurality of reception systems, and can improve the S/N ratio of a receive signal by properly processing a signal after demodulation.
In a wireless terminal device having a function of this receive diversity, the following receive diversity control can be performed. That is, for the purpose of power saving, the control is performed that the receive diversity function is switched between on and off as necessary. Using the property of having a plurality of reception systems, during communication by one branch, signal strength measurement is performed by the other branch in a frequency different from the former branch by switching receive diversity off.
Concerning such receive diversity, Japanese Laid-open Patent Publication No. 2006-324816 discloses that the object thereof is to decrease power consumption in a diversity receiver, and a condition decision circuit which decides whether specified conditions are met or not and a control circuit which temporarily stops diversity reception when the condition decision circuit judges that the specified conditions are met are included (Abstract, FIG. 1, etc.).
A wireless terminal device having a receive diversity function of W-CDMA will be described with reference to FIG. 1. This wireless terminal device 100 includes a transmit and receive antenna 102 and a receive antenna 104. A first reception system 106 and a transmission system 108 are constructed in the transmit and receive antenna 102, and a second reception system 110 is constructed in the receive antenna 104.
A transmit signal of a base station, that is, a downstream signal is received by the transmit and receive antenna 102 and the receive antenna 104. The received signal of the transmit and receive antenna 102 passes through a radio frequency-automatic gain control (RF-AGC) part 112, a cell detection-path detection part 114 and an inverse spreading-synchronous detection part 116, and amplification, cell detection, path detection, inverse spreading and synchronous detection are performed on the received signal thereof. The received signal is then added to a RAKE combination part 120 through a receive diversity (RxDiv) switch-over part 118. Similarly, the received signal of the receive antenna 104 passes through a RF-AGC part 122, a cell detection-path detection part 124 and an inverse spreading-synchronous detection part 126, and amplification, cell detection, path detection, inverse spreading and synchronous detection are performed on the received signal thereof. The received signal is then added to the RAKE combination part 120 through the RxDiv switch-over part 118.
When the receive diversity function is made to operate, the RxDiv switch-over part 118 allows the received signals of both of the first reception system 106 and the second reception system 110 to pass therethrough and to be RAKE-combined. When the receive diversity function is stopped, the received signal of the first reception system 106 only is added to the RAKE combination part 120.
For the RAKE-combined signal, SIR (Signal to Interference Ratio) determination is performed in an SIR determination-TPC (Transmit Power control) bit generation part 128. Thereby, a TPC bit pattern is generated. In a decoding-CRC decision part 130, a code is decoded from the received signal, and CRC (Cyclic Redundancy Check) decision is performed. A transmission data generation part 132 generates transmission data. A TPC bit pattern is added to a bit field along with another transmission data bit pattern, and transmitted from the transmission system 108 through the transmit and receive antenna 102 to a base station.
In communication between the wireless terminal device 100 having such a receive diversity function and a base station not depicted, FIG. 2 depicts a transmit and receive sequence when receive diversity is switched from an operating state (ON) to a stopped state (OFF), and FIG. 3 depicts a transmit and receive sequence when receive diversity is switched from a stopped state (OFF) to an operating state (ON). In FIG. 2, T1 is the period when RxDiv is ON and T2 is the period when RxDiv is OFF. In FIG. 3, T3 is the period when RxDiv is OFF and T4 is the period when RxDiv is ON.
In FIGS. 2 and 3, A is the downstream signal transmitted from a base station and B is an upstream signal transmitted from the wireless terminal device 100. The downstream signal includes a chain of continuous slots SLs and the upstream signal includes TPC bit fields. As described above, in the wireless terminal device 100, SIR measurement is performed for every received slot SL, and TPC bit setting is performed for each TPC bit field in the upstream signal. Thus, power control in response to variation of a communication state such as fading is executed.
When such receive diversity is switched from ON to OFF (FIG. 2), reception characteristics rapidly vary at a slot SLx just after the switching, and reduction of a receive gain and rapid power degradation arise. In this case, there is a possibility that demodulation errors increase and reception quality extremely deteriorates for a certain time.
When the receive diversity is switched from OFF to ON (FIG. 3), reception characteristics rapidly vary at a slot SLy just after the switching. In this case, a receive gain is enhanced and quality of service gets excessive for a certain time. In this case, there is a possibility that transmit power at the base station that is a resource of a network is excessively consumed, and vain power is consumed.
Concerning such problems, there is no suggestion or disclosure thereof in Japanese Laid-open Patent Publication No. 2006-324816, and no disclosure about solving means thereof is presented.